The present invention relates to the formation of ceramic matrix composites using precursor polymers which, upon pyrolysis or other energetic treatment, decompose to yield substantially pure refractory metal carbides and/or refractory metal borides.
Considerable effort has been devoted over the past 15 years to the development of effective methods for manufacturing ceramic matrix composites (CMC""s). Several approaches with potential for industrial use have been identified. The development of CMC""s with high temperature stability theoretically is possible; however, CMC""s have not yet been developed for use in extremely high temperature applications, such as multistage nozzles for rocket motors. Such nozzles must be capable of exhibiting high strength even after repeatedly withstanding temperatures of 1600xc2x0 C. and even higher.
Currently, multistage nozzles are made from tungsten and graphite, which have relatively high melting/sublimation pointsxe2x80x94a 3410xc2x0 C. melting point for tungsten, and a 3650xc2x0 C. sublimation point for graphite. The high temperature strength of a material is proportional to the melting point of that material. If CMC""s could be made using materials with higher melting/sublimation points than tungsten and graphite, then the resulting CMC""s should be effective alternative materials for making high temperature components, such as multistage nozzles.
Certain metal carbides and metal borides have melting temperatures even higher than the melting/sublimation points of tungsten and graphite. For example, hafnium carbide has a melting temperature of 3890xc2x0 C. and tantalum carbide has a melting temperature of 3880xc2x0 C. Metal carbides also exhibit desirable brittle to ductile transition temperatures in the range of 1725-1980xc2x0 C.
A CMC having a matrix of a refractory metal carbide and/or metal boride and comprising between about 20-30% particulate silicon carbide theoretically would be an ideal alternative for tungsten and graphite in multistage nozzles. Such metal carbides and/or metal borides also might be useful as high temperature coatings for other surfaces which are exposed to high temperatures during operation. In fact, the United States Air Force has recently initiated a new programxe2x80x94Integrated High Pay-Off Rocket Propulsion Technology (IHPRPT)xe2x80x94to incorporate such advanced materials into rocket and space propulsion systems.
Unfortunately, the most widely used method for making CMC""sxe2x80x94chemical vapor infiltration (CVI)xe2x80x94is slow, complex, and has many inherent difficulties. One major difficulty for high temperature applications is that CVI produces a CMC with substantial residual porosity (15-25%). The greater the porosity, the lower the strength of the CMC.
Polymer infiltration/pyrolysis (PIP) can produce a less porous CMC. However, PIP can only be used to make metal carbide/metal boride CMC""s if precursor polymers are developed which will decompose upon pyrolysis or other energy treatment to yield substantially pure metal carbides and metal borides.
The present invention provides a method for making a ceramic matrix composite comprising forming an infiltrated fiber reinforcement by infiltrating a plurality of plies of a fibrous material with a precursor polymer which decomposes to a substantially pure product selected from the group consisting of a refractory metal carbide and a refractory metal boride, and exposing the infiltrated fiber reinforcement to conditions effective to cure the precursor polymer and to decompose the precursor polymer to said substantially pure product.